Semiconductor integrated circuit devices are fabricated using various processes including, for example, lithography which involves transferring a pattern of a lithographic photomask (or reticle) into a photoresist layer which covers a wafer. In general, a lithography process comprises a sequence of steps including, for example, spin coating a photoresist layer on wafer surface, patterning the photoresist layer to form a photoresist mask, and performing an etch process to transfer an image of the photoresist mask into an underlying layer. The process of patterning the photoresist layer comprises exposing the photoresist layer to radiation (e.g., ultraviolet radiation, electron beam, x-ray, ion beams, etc.) through a lithographic photomask, and then applying an aqueous developer solution to the exposed photoresist layer to pattern the photoresist layer. For a positive photoresist layer, the portions of the photoresist layer that were exposed to radiation through the lithographic photomask are soluble in the developer solution and are etched away, whereas for a negative photoresist layer, the portions of the photoresist layer that were exposed to radiation through the lithographic photomask become insoluble in the developer solution while the non-exposed portions are soluble in the developer solution and are etched away.
Various techniques can be utilized to expose a photoresist layer through a lithographic photomask. For example, projection lithography is a common technique that is used to project an image of a lithographic photomask onto a photoresist layer (e.g., 1:1 projection or reduction projection). A lithographic photomask may comprise a transparent reticle substrate and an image defined by a patterned layer of optically opaque material formed on a surface of the reticle substrate. The lithographic photomask is mounted into a lithographic exposure tool, which may be integrated with a stepper tool, and radiation from a source of the lithographic exposure tool passes through the lithographic photomask and impinges on the photoresist layer, which results in transferring the pattern of the lithographic photomask into the photoresist layer after exposure and development. With the stepper tool, the lithographic printing process is performed step by step, wherein the lithographic photomask is aligned to a target area of the wafer (which may include a plurality of dies), followed by exposure of the target area, and then moved and aligned to another target section for exposure, etc.
In the manufacture of a semiconductor wafer, the integrated circuit features are formed layer-by-layer, wherein the features of one layer must be properly aligned to features in another layer. For example, when fabricating a back-end-of-line (BEOL) interconnect structure, metal lines that are formed in a given metallization level must be properly aligned to interlevel via contacts that are formed in a previous (underlying layer) to order to make proper connections between the metal lines and via contacts. Therefore, before lithographic exposure is performed to transfer an image of a lithographic photomask to a photoresist layer, the image of the lithographic photomask must be properly aligned to a previously defined pattern of a given layer of the wafer.
The alignment of a lithographic photomask is typically performed using alignment marks that are formed on the lithographic photomask and alignment features or marks that are formed in a layer of the wafer. In particular, a lithographic photomask to pattern a given layer of the wafer comprises an alignment pattern which has a predetermined relationship to alignment marks/features formed on a previous layer of the wafer. During an alignment process, the lithographic exposure tool is configured to visually locate the alignment marks/features formed on a given layer of the wafer, and adjust the position of the lithographic photomask to properly register the alignment pattern of the photomask to the alignment marks/features formed on the wafer.
This alignment process is problematic in instance where the layer(s) to be patterned on the wafer are “optically opaque” at the wavelength of the radiation source that is utilized by the lithographic exposure tool for visual alignment of photomasks, as a result of the type of materials and/or the thickness of the layer(s) to be patterned. In such circumstances, the lithographic exposure tool may be unable to properly view the alignment structures on the wafer which are covered by the optically opaque layer(s), thereby resulting in alignment error.